1. Field of the Invention
The present invention relates to a method and apparatus for diagnosing a pump system.
2. Background
Pumps are among the least reliable components in a process plant with the average Mean Time To Repair (MTTR) averaging two years. Recent advances in vibration sensor based condition monitoring are now routinely used to measure the vibration profile of a pump system to determine if large velocity or acceleration vibration levels are present. Such vibration levels are indicative of a failed or failing pump.
Pump maintenance is most often required due to operating a pump under conditions where bearing loads are high and where there is fluid induced damage to the impeller, i.e., cavitation and recirculation. The desirability of operating at the pump's Best Efficiency Point (BEP) is well known in the pump industry. At the BEP, bearing forces are designed to be at a minimum, vibration levels are lowest, and cavitation and recirculation are avoided. Examples of the impact on pump life due to off-BEP operation can be found in Pump Characteristics and Applications by M. S. Volk.
No commercial product, method or system has the ability to provide an operator or maintenance engineer with the actual operating pump performance curve and process operating point. An understanding of the operating point or range versus the intended operating range on a pump performance curve is key to operating a pump near its BEP and to diagnosing pump component damage when operating in off-BEP regions.
Vibration monitoring equipment such as that provided by Bently Nevada is well-known for condition monitoring of rotating equipment. The Bently Nevada system consists of sensors (typically accelerometers, displacement sensors, proximity velocity transducers and temperature sensors) that are appropriately mounted to rotary equipment such as turbines, compressors, fans, pumps and drive units such as motors.
Monitoring machine performance through vibration signature analysis is a practice spanning more than two decades. Many standards used for overall vibration measurements are based on specific rotational frequencies and integer multiples of these specific rotational frequencies. Vibration data is routinely collected either manually or with on-line systems from bearings on rotating machines. Bearing vibration measurements should include measurements in both the horizontal and vertical planes of each bearing. At least one axial vibration is made for each shaft.
Vibration readings on the bearing housing, using an accelerometer (acceleration) or a velocity transducer (velocity), provide sufficient data to detect the onset of bearing failure. Displacement or proximity probes measuring the motion the shaft relative to the bearing also provide useful data for diagnosing bearing failure. The motion of the shaft within the bearing as measured by the proximity sensor is commonly called an "orbit".
Rotating machines and pumps, by their very nature are dynamic machines. Vibration and proximity sensor data is also dynamic and is typically collected as trend data, FFT and waveforms. Most faults are identified by distinct frequency peaks or patterns and therefore frequency bands may be defined which bracket specific faults. These bands may be specifically scanned for amplitude changes which signal the need for further analysis. These scans will include comparing recorded vibration levels against alarm levels as well as a statistical analysis of variation and comparison to baseline values. The defined frequency bands will include the calculated or measured resonant frequencies of the major rotating machine mechanical components such as the shaft, impeller, radial bearings and thrust bearings. Analysis of vibrational data to identify known faults are further described in the CSI Application paper:
"Vibration Monitoring of Common Centrifugal Fans in Fossil Fired Power Generation".
Further analysis will include a review of the amplitude and phase versus frequency spectra, sometimes a referred to as the Bode plot, for the proximity and vibration sensors. Multiples of the machine component resonant frequencies, commonly termed harmonics, are also examined.
These known vibration monitoring techniques are applied in combination with the rotating machine performance curves to provide for root cause analysis the rotating machine in a method previously not performed.
The sensors are mounted for the purpose of detecting impending motor bearing or pump bearing failures through sensor signal analysis using conventional spectral analysis such as the Fast Fourier Transform (FFT).
The use of a vibration spectra is well known, but such use often requires a human expert to examine the spectra and deduce damage. Expert analysis is required since the frequency components for all of the mechanical components (bearings, impeller, piping, etc.) are all present at the same time. Therefore, discrimination of the vibration by component requires substantial skill. As vibration sensors provide a spectral output (frequency domain), the vibration peaks correspond to a multiplicity of failure modes that may be present at the same time. A single vibration spectra is likely to show the shaft frequency, impeller frequency, radial bearing fundamental frequency, thrust bearing fundamental frequency, motor harmonic frequency, pump/piping resonant frequency, mounting plate frequency, etc. In addition to these fundamental frequencies, the harmonics or multiples, and submultiples of these frequencies will also be present.
Traditional condition monitoring systems are used to detect damage that has already occurred to a rotating machine. The pump diagnostics method is able to detect rotating machine operating conditions that may lead to pump damage. The pump diagnostics method also provides an ability to focus the maintenance engineer or technician to examine a specific area of vibration spectra for evidence of a pending failure detected through knowledge gained from the pump performance signature.
Additional analysis is provided through the measurement of the pump to motor shaft alignment using position or proximity sensors and through the measurement of the shaft "orbit" within the bearing. Temperature measurements are strategically positioned to provide data on bearing "hot spots". Bently's system provides for off-line or "pseudo real time" acquisition of the above data and field processing of the sensor dynamic data which can be communicated to a centrally located display for viewing either via a proprietary communication or the Modbus protocol. Real time analysis has not been possible in the past due to bandwidth limitations in communications protocols.
Vibration sensors (piezoelectric accelerometers, velocity transducers and proximity sensors) are available from many suppliers such as Bently Nevada, Vibrametrics, Dytram and CSI and are often used with FFT algorithms for determination of vibration spectra for rotary equipment.
Vibration monitoring systems are available in portable "walk around" versions versus in situ systems where the vibration spectra is measured periodically. These portable vibration systems can provide for diagnosis of bearing failure, but are not as effective in determining the "root cause" for the bearing failure.
A diagnostics method is needed that provides for guidance in the repair of a failing pump, but most importantly, provides the operator with a root cause analysis that enables elimination of the cause of failure, which is often operation of the pump outside the BEP range. Available condition monitoring-only solutions provide limited guidance on elimination of the cause of failure as they observe the failure of the mechanical components, but do not identify the operating condition that caused the failure.
a. Electric Motor Diagnostics
Several manufacturers provide partial solutions for the off-line diagnosis of electric motors. Framatome and Liberty Technologies both provide for PC based electric motor diagnostics systems consisting of vibration sensors to detect motor bearing failure, a measurement of motor voltage wherein current and phase are coupled to an FFT for the calculation of a motor signature, and temperature sensors to sense elevated motor winding temperatures and, in some cases, flux analysis, and insulation characteristics of the motor windings and shaft alignment sensors to measure motor-to-coupler alignment.
CSI has recently announced a motor diagnostic system that provides data logging of key motor diagnostic attributes that are manually collected and uploaded to a PDM device at regular intervals.
Siemens has a smart motor system that provides pseudo real-time diagnostics information such as that summarized above via a sensor system connected to a field mounted motor analysis computer.
Current motor diagnostics systems capture only motor and motor power related diagnostic information such as power (P), voltage (V), current (I), phase (.O slashed.), and flux (f). Current systems do not have the ability to look at the influence of the rotating machine (load) and its influence on the motor.
b. Alignment Systems
The need to provide for precise alignment of the pump and driver source through a coupling means or intermediate gear box is well known. A number of suppliers exist for the systems to facilitate optimum alignment of the pump to power source. Such systems may include traditional dial indicators, electronic position sensors and most recently laser alignment systems such as the ones manufactured by Vibra Align Inc. and Ludeca, Inc. These systems provide assistance to the pump technician for the installation or validation of proper motor and driver alignment with the pump. Many alignment systems provide an electronic output that can be displayed at the pump in the field.
c. Pump Diagnostic Systems
Pumps are one of the least reliable devices in a process plant. The proper pump selection and application, installation, use and maintenance must be assured for long life.
Pump manufacturers such as Gould Pumps commonly provide a pump performance curve with each pump. A pump curve is intended to assist a user in properly selecting the correct pump and pump impeller as well as to assist a user in operating a pump in the most efficient manner while producing the desired flow and pressure (head). Pumps are often used with constant speed power sources such as a 1800 RPM electric motor, or with a variable speed drive (VSD) where the pump speed can be changed to vary the pump flow and head output. When pumps are used with variable speed drives, distinct pump performance curves can be provided by the manufacturer at each desired speed. However, such curves are often calculated using the pump affinity law from the originally provided pump curve. The pump affinity laws are well known and are described in Yedidiah, Centrifugal Pump User's Guidebook.
Several manufacturers use commercial personal computer systems for the measurement and calculation of the pump performance curve at the factory for new or repaired pumps. These systems are sometimes used by pump manufacturers to calculate and provide the pump performance curves for the end user. These systems may include sensors for determining pump shaft speed (RPM), inlet and outlet pressures, outlet flow, shaft horsepower (brake horsepower) and fluid temperature. Standard algorithms, such as those described by Yedidiah, author of Centrifugal Pumps User's Guidebook, are applied to provide the pump performance curve.
MARINTEK has undertaken work for the development of a knowledge-based diagnostics system called ROMEX, which is designed for rotating machines. The ROMEX system is a PC-based system which integrates data from commonly used condition monitoring systems and covers mechanical and performance related faults with coverage of the rotating machine rotor, stator, coupler, bearing, blades, aerodynamics and combustion chamber for gas turbines. The ROMEX system does not use a pump performance curve as a method for diagnosing possible off-BEP operating conditions or changes in the pump performance curve as a primary source of diagnostics for pump maintenance.
d. Pump Sensor Fusion
Recent published research from the Colorado School of Mines and their SHARP system (System Health Assessment and Real-Time Prediction) suggest that a diagnosis of pump health can be made via the fusion of physical variables such as pump inlet pressure, outlet pressure and flow in conjunction with a large artificial intelligence system made by Gensym. Artificial intelligence expert systems are used with some maintenance systems and often involve hundreds and even thousands of rules necessitating a large and expensive computing workstation. The SHARP system does not use the pump performance curve as a primary source of diagnosis for pump maintenance.
e. Pump Field Diagnostics Systems
Ingersoll-Dresser Pump has a remote pump monitoring system tradenamed PumpTrac.RTM. which provides for the collection of vibration data, physical process variables including pressure, flow, temperature and motor amperage. The monitoring systems provide for the trending and data logging of input variables, an alarm mode for each variable and a phone modem connection for alerting a plant operator of an alarm. The system is able to provide pump variable monitoring for up to eight pumps.
Ingersoll-Dresser's PumpTrac Remote Pump Monitoring System has a hardened pump diagnostics system with I/O that displays local trending data of the input sensors, typically pump inlet and outlet pressures, process and gearbox oil temperatures, flow and vibration condition monitoring sensors. The display provides trend displays for each variable with the ability to display multiple windows so visual correlation of process variable trends with vibration can be made.
Further, the Ingersoll-Dresser system has the ability to set soft alarm points that can be actuated when an alarm point is exceeded. In one option, a soft alarm can actuate a traditional modem built into the system to call a predetermined number to indicate what variable has been exceeded with data messages or a pre-recorded message.
The Ingersoll-Dresser device requires AC power and a dedicated telephone line. The system is not networked with process industry standard protocols and the use of AC power prevents certification to industry electrical intrinsic safety standards.
The Ingersoll-Dresser PumpTrac system provides the variables needed to establish a pump performance curve described in the present application, but does not provide the pump curve. However, the PumpTrac system does not provide for use of the secondary performance curves which provides a basis for root cause analysis of pump component failures, which is described in the method herein.
f. Field Diagnostics Systems
Field diagnostics systems are known for air operated valves and motor operated valves such as commercial systems available from Framatome, ABB, Liberty Technologies and Fisher Controls. There is no known field diagnostic for rotating equipment.
g. Point Diagnostics Devices
Several manufacturers provide sensors that can be applied to a pump system for the partial determination of pump health. These sensors may measure the corrosion of the pump casing based on a thickness measurement through ultrasonic thickness detection systems such as those manufactured by Stresstel, or by corrosion sensors such as those manufactured by Diagnetics. Similar portable monitoring devices, such as manufactured by Vibrametrics, provide point measurements of pump casing corrosion and thickness.
Pump and gearbox oil contamination and breakdown are commonly known problems with pump systems. In situ measurements of oil conditions can be provided by devices such as the Digital Contamination Alert particle counter provided by Diagnetics or through taking oil samples for off-line laboratory analysis.
High bearing stress resulting from operation outside said design regime will lead to bearing degradation. Bearing degradation can be detected via vibration monitoring. If degradation is severe enough, bearings will exhibit wear, which can be detected by oil sample analysis for wear particles.
None of these techniques identify improper operation which results in equipment stress leading to progressive damage, and ultimately, failure.
h. Need For Root Cause Analysis Method and Apparatus
Consequently, there is needed a rotating equipment diagnostic method and apparatus that identifies the operating conditions which create damaging stress to said equipment.
Further, there is needed a diagnostic method and apparatus that is effective in determining the root cause for bearing failure of rotating equipment. Additionally, there is a need for a diagnostic method that provides guidance in the repair of a failing pump by supplying the root cause analysis to an operator. By supplying root cause analysis to an operator, the diagnostic method enables elimination of the cause of failure.
Further, a system that has the ability to look at the influence of a load from a rotating machine.
A system is further needed that uses a pump performance curve as a method for diagnosing possible operating conditions outside of the recognized, recommended operating design regime or BEP.
Still further, there is a need for a system that does not require large and expensive artificial intelligence computing workstations yet still accomplishes the forgoing function.
There is further a need for a second level of pump component diagnosis that provides an ability to conduct root cause analysis of a pump or rotating equipment failure or an operating condition responsible for the failure.